[Bugfix] Add LightningOptimizer parity test and resolve AMP bug

[tchaton]

What does this PR do?

Fixes #5165
Fixes #5159

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[awaelchli]

@tchaton you can move the if statement inside the to_lightning_optimizer function to achieve idempotence and avoid code duplication.

[awaelchli]

the tests are a bit complex, I don't really understand them but I was able to verify that they fail on the master branch with the wrong loss values so I suppose the scaling bug is fixed 👍

[tchaton]

the tests are a bit complex, I don't really understand them but I was able to verify that they fail on the master branch with the wrong loss values so I suppose the scaling bug is fixed 👍

The test are performing parity comparison with vanilla training in several scenarios and making sure everything matches properly when using enable_pl_optimizer=True/False. We will need to add more parity test such as Apex, DDP modes, multi optimisers.

Best,
T.C